[CI] Fix website build

[barry-jin]

Description

Sphinx has upgraded to v4.0. This introduced some issues in website build. This PR will try to fix these issues. The alternative way is to freeze sphinx package to v3.5.4

Checklist

Essentials

• [ ] PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• [ ] Changes are complete (i.e. I finished coding on this PR)
• [ ] All changes have test coverage
• [ ] Code is well-documented

Changes

• [ ] Feature1, tests, (and when applicable, API doc)
• [ ] Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

cc @dmlc/gluon-nlp-team

[github-actions[bot]]

The documentation website for preview: http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR1573/60f93c0d2cf677914185e643abd675c732490368/index.html